The spinal column surrounds and protects the spinal cord and is a column made of up bones, discs and ligaments. The spinal column is made up of 24 bone segments (7 cervical, 12 thoradic, and 5 lumbar spinal vertebrae), plus the sacrum and the tailbone (coccyx), and provides the supporting structure of the spine. Nerves branch out from the spinal cord and pass through openings between the vertebrae to obtain sensation from and control the movements of our body parts.
Discs are located between the bone segments (vertebrae) and function as “shock absorbers” and give the spine flexibility to move and bend. The outer part of the disc is called the annulus fibrosus (annulus), which is a strong structure that surrounds and supports an inner jelly material called the nucleus pulposus (nucleus). Disc material in the nucleus is mainly composed of water and proteins (collagen) and as the body ages, the water content decreases as the collagen content increases. This causes the disc to be more susceptible to degeneration (or flattening) wherein the inner jelly material (nucleus) can bulge and press against the annulus fibrosus. This can stimulate pain receptors, causing pain to occur. With age, the disc is also more susceptible to injury such as tears in the annulus that can lead to a condition known as a herniated disc, wherein the nucleus of the disc escapes the annulus and may then press upon adjacent nerves or the spinal cord. If uncorrected, this disc herniation can lead to excruciating pain, deformity, and neurological and musculoskeletal dysfunction. While non-surgical options such as rest, heat, pain medications and physiotherapy are available to patients with disc degradation or injuries, these conditions sometimes require surgical intervention.
Of interest with respect to disc degradation or injuries, it should be understood that prior to the 1900's, treatment options for osteoarthritic degenerative joint disease, such as that of the hip and knee, were limited. Surgeons performed limited procedures to relieve terrible pain, including joint debridements, nerve division, osteotomy, and if all else failed, arthrodesis (fusion) of the diseased joint. While hip and knee fusion were successful in terms of pain relief from the diseased joints, it placed substantial non-physiologic strain on surrounding joints as well as the spine, typically in an individual already beset by degenerative joint disease. This resulted in a cascade of joint failures in both legs and hips, and further pain. In the 1960's, the advent of hip arthroplasty by Sir John Charnley and subsequent knee arthroplasty in the early 1970's ended the need for most patients to undergo such long bone arthrodesis, marking a substantial improvement in the quality of life surgeons could offer to patients with degenerative joint disease. But, while hip and knee procedures were improved, procedures for dealing with spine disorders have been slower in development by medical professionals.
Up until the last few years, anterior cervical discectomy and arthrodesis were the standard surgical methods for dealing with disc degradation or damaged vertebral bodies. In the cervical spine, anterior cervical discectomy involves the removal of the entire diseased spinal disc to relieve the lower back or leg pain associated therewith. The disc is replaced with a bone graft, and the entire complex of the graft and the adjacent vertebral bodies is then secured with a plate and screws, allowing the complex to then fuse over the ensuing months following the surgery. Unfortunately, this fusion of vertebral bodies result in redistributed spinal loading stresses and can accelerate degenerative processes in the cervical discs above and below.
Arthrodesis (or spinal fusion) is also performed to address other spine problems, such as vertebral body fractures or scoliosis. As shown in FIGS. 1A and 1B, when spinal fusion SF is performed, the second cervical vertebra V2 is removed (corpectomy), with discectomies performed above and below the resected vertebral body. A bone graft BG is then inserted into the void between the first V1 and third V3 vertebrae and a plate PL is positioned over first V1 and third V3 vertebrae and bone graft BG that lies in between. Screws SW are used to secure plate PL to first vertebra V1, intermediate bone graft BG, and third vertebra V3 so that the first and third vertebrae and bone graft are mechanically secured. With time, bone fusion will occur between the first vertebra, bone graft, and the third vertebrae, and the entire segment will act biomechanically as a single long bone segment.
While 50% to 94% of patients have good to excellent results immediately after arthrodesis, adjacent motion segment(s), specifically the intervertebral discs, are lost in this process. This results in a non-physiologic spine that distributes stress unevenly and results in greater wear and tear in adjacent segments. The remaining motion segments immediately above and below the long fusion mass experience a disproportionate degree of biomechanical stress, and are at greater risk of developing instability in turn, a condition also known as “adjacent segment disease”. Studies indicate that up to 30% of patients who undergo single-level spine fusion require reoperation to treat disc degeneration at the next level either above or below the original fusion within ten years. It became evident that a need existed for a new system to treat spinal disease in a manner that avoids fusion of movement segments.
With knowledge of the disadvantages of spinal fusion and the advantages of hip and knee arthroplasty, it was inevitable that artificial hip and knee technology would be translated to artificial disc designs for spinal arthroplasty. Such research has proliferated over the last decade, with the development of multiple approved artificial disc designs in Europe.
When dealing with artificial disc concepts, those of skill in the art generally divide prosthetic discs into two major categories: nucleus replacements and total disc replacements.
Nucleus replacements are designed for use when the major feature of the degenerative process involves the nucleus pulposus but has spared the annulus and supporting structures. Consequently, nucleus replacement is useful only during the early stages of disc degeneration where minimal segment collapse has occurred. Nucleus replacement requires creating a hole in the annulus to insert the prosthetic nucleus. Since the prosthetic nucleus is unattached inside the annulus, it is sometimes expelled from the annulus through the hole.
An example of a nucleus replacement is the PROSTHETIC DISC NUCLEUS (available from Raymedica of Bloomington, Minn.) generally shown as PDN in FIG. 2A. PDN is an implant with a hydrogel core C wrapped in a woven polyethylene cover. A dehydrated spacer, it expands as it absorbs water after implantation. The size of the implant varies and it can be placed through an anterior or posterior approach. For proper balance it should be used in pairs. Outcomes of a cadaveric study of the biomechanics of this implant were favorable. The cadaveric model, however, failed to permit full hydration of the device. Since 1996, 423 patients have received the PDN with a surgical success rate of 90% and clinical results have been encouraging. The main problem of a 10% implant migration rate, has led to a series of modifications to the procedure for installation.
Another example of a nucleus replacement includes the AQUARELLE STRYKER (available from Howmedica of Rutherford, N.J.) (not shown) which is a hydrogel material based on polyvinyl alcohol that is hydrated before implantation. This material can be implanted through a tapered cannula placed in an annulotomy via a posterior or lateral approach. Finally, the PROSTHETIC INTERVERTEBRAL NUCLEUS (available from Raymedica of Bloomington, Minn.) (not shown) is another example of a nucleus replacement. It is a polyurethane material instilled into a balloon that is inserted into a disc space. Once placed into the disc space, the chamber is filled with the material, which then cures in situ.
In contrast to nucleus replacement, total disc replacement addresses degenerative processes throughout the intervertebral disc. It represents a surgical procedure similar to that required with anterior cervical discectomies and corpectomies. These prosthetics are designed to restore the normal movement of the diseased motion segment. Using technology gained from orthopedic joint replacements, numerous strategies have been devised to create a successful prosthetic disc. Generally, prosthetic discs comprise a polymer center bounded by metallic end-plates with prongs or spikes to abut against the surrounding bone segments to prevent motion. Elastomers, viscous fluids, fluid-filled chambers, and articulating components have been used in prosthetic discs. Strength, durability, biomechanical and biochemical compatibility, ease of implantation, and the immediate postoperative and long-term stability of the device must be considered and will affect the utility of a replacement disc.
In the lumbar spine, the LINK SB CHARITÉ III (available from Waldemar Link GmbH and Co. of Hamburg, Germany), PRODISC (available from Aesculap AG and Co. of Tuttlingen, Germany), and ACROFLEX-100 prosthesis (available from DePuy AcroMed of Raynham, Mass.) are the most widely implanted total-disc replacements (see FIGS. 2B-2D, respectively).
In 1984, Drs. Karin Büttner-Janz and Kurt Schellnack began to work with the Waldemar Link GmbH & Company (Hamburg, Germany) to develop a replacement lumbar disc. After several design iterations, the final version of the disc, the LINK SB CHARITÉ III (generally designated LSBC in FIG. 2B) is currently available in Europe. The device is composed of an ultrahigh molecular weight core C sandwiched between two cobalt-chromium alloy endplates EP. More than 3000 of these discs (all generations) have been implanted throughout Europe. In 1994, a multicenter retrospective review of the early clinical results with the LINK SB CHARITÉ III lumbar artificial disc was published. Pain reduction, mobility, walking distance, and strength improved significantly in 93 patients when compared to their preoperative condition. The complication rate, as a result of disc migration or dislocation and device failure, was 6.5%. In another study by others of the LINK SB CHARITÉ III disc, forty-six patients were studied a mean of 3.2 years after implantation: 63% reported satisfactory results. The success rate was 69% in patients who underwent isolated disc replacement and 77% in patients who had undergone previous back surgery. Two patients had the prosthesis removed. Seven patients underwent posterolateral fusion without removal of the device.
The PRODISC (generally designated PD in FIG. 2C), developed by Dr. Thierry Marnay, consists of two CrCoMo alloy endplates EP covered with titanium plasmopore to enhance osteointegration (FIG. 2C shows one endplate of the PRODISC artificial disc). The inferior articulation is a monoconvex surface that slides into a concave upper rim. This design permits the implant to be inserted with significantly less distraction of the disc space than the LINK SB CHARITÉ III disc. Because movement is across two surfaces, the motion is semiconstrained. The clinical outcomes associated with the PRODISC have been presented at several meetings, but published data are limited.
Another study has reported experience with the ACROFLEX-100 artificial lumbar disc (generally designated AF in FIG. 2D). This prosthesis consists of two titanium endplates EP vulcanized to a polyolefin rubber core C. The disc was implanted in six patients via a midline transabdominal approach, four of whom reported satisfactory results. In one patient, the rubber core fractured and a revision was necessary.
In the cervical spine, the CUMMINS/BRISTOL disc (available from Medtronic Sofamor Danek of Memphis, Tenn.) (not shown) and the BRYAN CERVICAL DISC prosthesis (available from Medtronic Sofamor Danek of Memphis, Tenn.) (generally designated BCD in FIG. 2E) are under investigation. The CUMMINS/BRISTOL disc consists entirely of stainless steel or titanium and is of ball and socket construct. Because it is screwed into place, this artificial disc can only be used for single-level disc disease. The BRYAN CERVICAL DISC BCD is a composite-type disc prosthetic designed with a wear resistant, low friction, elastic nucleus C and two anatomically shaped metal plates EP on either side of the nucleus. In Europe, the BRYAN CERVICAL DISC prosthesis has been implanted in 97 patients, and has had favorable results for the purpose of disc replacement only.
These discs represent the first generation of spinal arthroplasty devices and a number of concerns have arisen from the possible use of these artificial discs. One disadvantage of prior art prosthetic discs involves the interface that exists between the prosthetic and the adjacent vertebrae structure. Wear or “micro-motion” can cause debris to accumulate either within the prosthetic disc or the artificial joint space. Additionally, one of the greatest concerns of prosthetic disc replacement is that of implant migration. While more leeway for implant migration exists in the lumbar spine, disc migration can have grave consequences in the cervical spine. Artificial disc migration can result in devastating neurological injury, so disc migration prevention is paramount in importance.
While disc arthroplasty using artificial discs may help solve the biomechanical problems associated with spine fusion, this technology primarily benefits patients who have disc disease. Even if artificial discs are approved in the US, they would not help those patients with non-disc related forms of spinal disease, such as spinal fracture or scoliosis, that currently require spinal fusion. Also, there exist numerous disadvantages of using artificial discs exclusively as discussed hereinabove. Therefore, there remains a long-felt need for a spine replacement system that can address all forms of subaxial spine and disc disease, that recreates the anterior and middle columns of the subaxial spine complete with adjacent motion segments, and that does not possess the disadvantages of prior art prosthetics.